In recent years, hearing aids and their associated battery power supply and amplifier have been successfully miniaturized to a size where they can be worn entirely within the ear. A persistent problem with the state of the art units of this type is the distortion-power consumption trade-off. More specifically, low distortion units run as conventional Class A device circuits can be designed to provide suitably low output signal distortion, particularly at low levels; however, since class A devices (and in particular class A output stages) exhibit a very low power efficiency, the concomitant battery drain by such units is quite high, requiring frequent replacement of the hearing aid battery. In an effort to improve the overall power efficiency of the system, resort has been made to units employing class B amplifier circuitry in the output stage. Since class B amplifiers, by definition, idle at a cut-off condition, the usual result in simple class B circuits is that, at low signal levels, distortions are encountered which render the signal quality undesirably harsh.
To avoid this problem rather elaborate balancing circuitry is required to equalize the behavior of the two halves of the amplifier circuit at both high and low drive levels, and to provide uniform crossover characteristics. In practice this mandates the use of strong inverse feedback which, as is well known to those knowledgeable in the art, requires high gain circuitry with strong inverse feedback applied therearound. Because of this, there is a strong possibility of oscillation, which in turn frequently requires the use of physically large feedback or decoupling capacitors of several microfarads.
A superficially attractive, but to date never satisfactorily accomplished, alternative is to use a class D amplification system of pulse width modulation, wherein a ultrasonic rectangular waveform generator has its duty cycle continuously adjusted responsively to the instantaneous amplitude of the audio waveform received from the hearing aid receiver. The resulting output contains the original audio frequency intelligence, as well as switching modulation components in the supersonic range. Although such systems are characterized by low distortion and are power-efficient, nevertheless a significant amount of battery power is represented in losses contained in the ultrasonic switching modulation components. Moreover, such systems are usually more complex than straightforward class A or class B systems, owing to the necessity for providing an oscillator, a switching modulator and a variety of adjustable components necessary to secure adequate balance of the circuit. A further source of difficulty is posed by the requirement that it be powered by a system battery of 1.5 volts.
To the applicant's knowledge, no adequately small low-power, low-distortion class D amplification system has been devised which meets all system requirements, and which can additionally be manufactured in a sufficiently small size to allow fabrication of a hearing aid designed to fit within the ear. Thus, such an amplifier would be a useful and novel addition to the art, particularly if characterized by tolerable battery drain, small size and low distortion characteristics.